1. Field of the Invention
The invention relates to a flat wiring member (or a flat wire harness or a flat harness) and, in particular, to a flat wiring member that is used for controlling a secondary battery module and is advantageous for increasing length and reducing the cost, as well as a method of manufacturing the flat wiring member.
2. Description of the Related Art
In recent years, secondary batteries typified by lithium-ion batteries and nickel-hydrogen batteries have been used more widely from portable electronic devices (e.g., mobile personal computers or cell-phones) to large electrical equipments (e.g., power sources for vehicles such as HEV (hybrid vehicle) or EV (electric vehicle), power storage sources, uninterruptible power-supply systems of data centers, etc., and electric-load leveling systems for natural energy of sun or wind, etc.).
When used in large electrical equipments, secondary batteries are required to have much higher output and capacity than in portable electronic devices. Thus, it is necessary to increase capacity of each secondary battery as a single cell (hereinafter, sometimes referred to as “cell”) as well as to combine plural batteries.
When using secondary batteries in a large electrical equipment, a secondary battery module is generally formed by coupling plural cells using connecting members such as bus bars. In case of a secondary battery module in which cells are connected in series, variation in battery characteristics among the cells, if any, causes restriction of battery characteristics or reliability of the entire secondary module because of the cell having the poorest battery characteristics. Therefore, it is important to suppress variation in battery characteristics among the cells.
In addition, when secondary batteries (especially, lithium-ion secondary batteries) are in an overcharged state or in an over-discharged state, battery performance significantly deteriorate or battery life is extremely shortened. Thus, when charging or discharging, highly accurate voltage control (e.g., at several tens of mV) is required to prevent overcharge or over-discharge. For controlling voltage, an electrode of each cell is connected to a control circuit or a protection circuit via a monitor wiring member.
In conventional secondary batteries for portable electronic devices, flexible printed circuit (FPC) boards, etc., having a predetermined wiring formed thereon are preferably used as a monitor wiring member for space saving and easy assembly (also for prevention of miswiring). On the other hand, in secondary battery modules for large electrical equipments, capacity of each cell and the number of cells are increased as described above. Accordingly, the number of wirings on the monitor wiring member needs to be increased and each wiring becomes much longer (e.g., about 0.5 to 1 m in length) than that in portable electronic devices.
In general, for manufacturing a FPC, a copper-clad laminate having a thin copper foil laminated on a flexible resin base material (e.g., a polyimide film) is used and the copper foil is patterned (e.g., by photolithography and etching). After the patterning, an excess portion of the flexible resin base material of the copper-clad laminate is often removed by punching or cutting.
Here, there is a problem when using a FPC as a monitor wiring member of secondary battery module for large electrical equipment since existing photolithography equipments are intended to handle the size up to about 12 inches and thus cannot directly process the size of such FPC (e.g., about 0.5 to 1 m). In addition, the FPC also has a problem that an increase in wiring length causes an increase in wiring resistance since the copper foil (i.e., the wiring) is very thin (e.g., about several tens of .mu.m). Note that, increasing width of each wiring in order to suppress such an increase in wiring resistance is to run counter to space saving of monitor wiring member and is not a realistic solution in view of size restriction caused by the photolithography equipments.
Various flat harnesses have been proposed to solve such problems. For example, JP-A-2002-157924 discloses a flat harness provided with a lower insulating film having an adhesive layer on an upper surface, conductive wires laid along a predetermined wiring pattern on the upper surface of the lower insulating film and an upper insulating film covering the upper surface of the lower insulating film having the conductive wires laid thereon, wherein the conductive wires are plural solid wires laid on the same surface and the plural solid wires are arranged in tight contact with each other on the upper surface of the lower insulating film so that adjacent solid wires are in parallel. According to JP-A-2002-157924, by using a solid wire as a conductor and shaping/wiring the solid wires along a predetermined wiring pattern, it is possible to provide a flat harness which realizes a thinner shape, larger current capacity and lower manufacturing cost than that using a twisted wire as a conductive wire.
Meanwhile, JP-A-2002-203431 discloses a flat harness provided with a first flat cable and a second flat cable connected to a middle portion of the first flat cable at a connecting portion and used for lower current than the first flat cable, wherein at least some conductors of the first flat cable are electrically connected to at least some conductors of the second flat cable. According to JP-A-2002-203431, since the first flat cable (flexible flat cable or ribbon cable, etc.) having a large conductor cross sectional area and suitable for high current is used for connection to a high-current circuit and the second flat cable (FPC, etc.) suitable for low current is used for connection to a low-current circuit, an increase in wiring width for suppressing an increase in wiring resistance is not required and it is thus possible to reduce the overall width of the flat harness as compared to that using FPC for all wiring portions.
In addition, JP-A-2009-104889 discloses a vehicle wire harness which is arranged in a vehicle and in which plural enamel wires bundled together are hardened in a state of being firmly fixed to each other by an adhesive or a bonding material formed of an insulating resin and the plural enamel wires in the hardened state have a two-dimensional shape or/and a three-dimensional shape corresponding to the layout in the vehicle. According to JP-A-2009-104889, since a conductor formed of a single-core wire is used and an insulation layer of the enamel wire is thin, it is possible to reduce diameter and weight of the wire harness.